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Observation ofγvibrations and alignments built on non-ground-state configurations inDy156

Physical Review C2015Vol. 91(3)

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Abstract

The exact nature of the lowest ${K}^{\ensuremath{\pi}}={2}^{+}$ rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the \ensuremath{\gamma} degree of freedom perpendicular to the nuclear symmetry axis. Very few such \ensuremath{\gamma} bands have been traced past the usual backbending rotational alignments of high-$j$ nucleons. We have investigated the structure of positive-parity bands in the $N=90$ nucleus $^{156}\mathrm{Dy}$, using the $^{148}\mathrm{Nd}(^{12}\mathrm{C},4n)\phantom{\rule{0.28em}{0ex}}^{156}\mathrm{Dy}$ reaction at 65 MeV, observing the resulting \ensuremath{\gamma}-ray transitions with the Gammasphere array. The even- and odd-spin members of the ${K}^{\ensuremath{\pi}}={2}^{+}$\ensuremath{\gamma} band are observed up to ${32}^{+}$ and ${31}^{+}$, respectively. This rotational band faithfully tracks the ground-state configuration to the highest spins. The members of a possible \ensuremath{\gamma} vibration built on the aligned yrast $S$ band are observed up to spins ${28}^{+}$ and ${27}^{+}$. An even-spin positive-parity band, observed up to spin ${24}^{+}$, is a candidate for an aligned $S$ band built on the seniority-zero configuration of the ${0}_{2}^{+}$ state at 676 keV. The crossing of this band with the ${0}_{2}^{+}$ band is at $\ensuremath{\hbar}{\ensuremath{\omega}}_{c}=0.28(1)\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ and is consistent with the configuration of the ${0}_{2}^{+}$ band not producing any blocking of the monopole pairing.

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Abstract

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